Photoelectric measurement

Monocular spatial attitude measurement method guided by two dimensional active pose
Liu Feng, Guo Yinghua, Wang Lin, Gao Peipei, Zhang Yuetong
2024, 53(2): 20211026. doi: 10.3788/IRLA20211026
[Abstract](872) [FullText HTML] (212) [PDF 2175KB](195)
  Objective  Monocular vision measurement technology has the advantages of simple structure, low cost, convenient and flexible operation, and there are two types of monocular vision measurement technology in general. One is the combination of monocular camera and measured object, but it needs to design a suitable cooperative target, which has certain limitations. The other is the combination of monocular camera and active sensor, but the adjustment or calibration process of the pose relationship between the camera and the active sensor is more complicated. Aiming at how to quickly measure the pose of space objects, this paper studies a monocular visual spatial pose measurement method based on two-dimensional active pose guidance. This method only requires a camera and a precision two-dimensional carrier to collect one image before and after the carrier rotates, which can complete the rapid attitude measurement of space objects. The attitude measurement method has the advantages of low cost, simple operation and large measuring range, which is less dependent on equipment.  Methods  A monocular attitude measurement system composed of monocular camera, precision two-dimensional platform and measured object is established. And an attitude measurement model of monocular camera, precision two-dimensional platform and tilt meter is designed. A precision checkerboard image and two angles of the two-dimensional platform under different image positions were taken by the camera for multiple times to carry out joint visual calibration of the camera and the two-dimensional platform (Fig.2). The pose relationship between the camera and the platform was obtained, and the pose relationship between the checkerboard and the initial camera coordinate system was calculated. Based on the coordinate system of the geodetic inclinometer, the pose relationship between the inclinometer and the attitude measuring system was calibrated according to the coordinate system relationship between the inclinometer and the checker (Fig.3), and the measured values were converted to the coordinate system of the inclinometer, realizing the rapid measurement of monocular vision.  Results and Discussions  A monocular visual spatial pose measurement method based on 2D active pose guidance is studied. Through the acquisition of precision checkerboard images for many times, the pose relationship between the camera and the two-dimensional platform, the inclinometer and the measuring attitude system was obtained, and the calibration errors of pitch angle and roll angle were both < 0.31° (Fig.7). Taking the checkerboard as the measured object, combined with the calibrated parameters, the measurement error is the largest when the pitch angle is about 15°, and the measurement error is 0.82°. When the roll angle is about −15°, the maximum measurement error is −0.43° (Fig.10).  Conclusions  In this paper, a monocular visual spatial pose measurement method based on 2D active pose guidance is studied, and the attitude measurement model of monocular camera, precision 2D pedestal and inclinometer is established. This method uses only one camera, and does not need to consider the baseline distance under binocular setting. Moreover, this method can realize the rapid measurement of the object's attitude after calibration, and realize the measurement of the object's attitude under fixed-axis dual-angle photography. The experimental results show that the proposed method can be used to measure the attitude of space objects quickly.
Displacement measurement error of grating interferometer based on vector diffraction theory
Lei Lihua, Zhang Yujie, Fu Yunxia
2024, 53(2): 20230536. doi: 10.3788/IRLA20230536
[Abstract](29) [FullText HTML] (7) [PDF 1629KB](7)
  Objective  The grating interferometer displacement measurement system, as one of the most precise measuring instruments in the measurement field, is not able to ensure the perfect assembly of the grating's attitude position during the measurement process, which makes the deviation between the grating's grating vector direction and the motion vector direction, leading to the periodic nonlinear error in the displacement measurement results. In previous studies, the assembly error of grating displacement measurement system is usually unfolded in the scalar state, ignoring the effect of incident azimuth angle on the system. Based on the grating vector diffraction theory, this paper analyses the attitude position error between the grating and the displacement stage as well as the readhead that occurs during the displacement measurement of the grating interferometer, and illustrates the possible displacement measurement error by analysing the amount of angular deviation of the three dimensions, so as to provide the theoretical basis for the improvement of the subsequent device.  Methods  Ideally, the displacement measurement of a grating interferometer is based on the period of the core component, the grating. But due to the non-ideal assembly of the grating, the displacement stage, the readhead, the optics, and other system modules, there will be geometric errors in the system. The non-ideal assembly of the grating and the displacement stage, as well as the non-ideal assembly of the grating and the readhead, are the main factors leading to the geometrical error of the system. In this paper, we analyse the attitude position error between grating, displacement stage and readhead which occurs during the displacement measurement of grating interferometer, by establishing displacement coordinate system OXYZ and grating coordinate system OX'Y'Z', and referring to the attitude representation method of aircraft in the field of inertial navigation. We set the roll, pitch and yaw angles of one-dimensional grating to be α, β and γ respectively, which are common in describing the assembly state of the 1D grating relative to the translation stage. By analysing the amount of angular deviation in the three dimensions based on the grating vector diffraction theory, the possible displacement measurement errors are analysed and illustrated.  Results and Discussions  The results of the analyses of the grating assembly errors show that the geometrical errors caused by the non-ideal assembly of the metrology grating are mainly due to the rotational error angles β and γ around the Y' and Z' axes, while the rotation of the grating around the X' axes does not cause any additional measurement errors. It can be seen from the error expressions that error angles β and γ have the same effect on the measurement error. When analysing the readhead assembly error, it was found that the biggest difference between this and the encoder assembly error is that the readhead assembly error causes the system not to satisfy the Littrow structure, further complicating the problem. However, because of this, it explores a more general conclusion based on the generalised one-dimensional grating equations in this paper, from which the relationship between the systematic measurement error and the three error angles α, β and γ and the angles θ1, θ2, Ψ1, Ψ2 affecting the relative states of the incident P-light and Q-light is discussed respectively.  Conclusions  This paper analyses the measurement errors caused by clamping problems when using the grating displacement measurement system from two aspects of grating assembly errors and readhead assembly errors, and provides an analytical description of the possible displacement measurement errors. In the grating interferometer displacement measurement system, the analysis based on the grating vector diffraction theory shows that, when the Littrow incidence structure is satisfied, among the roll angle, pitch angle and yaw angle, the roll angle has no effect on the measurement results, and the expressions for the effects of pitch angle and yaw angle on the displacement measurement results are derived; When the Littrow incidence structure is not satisfied, the oblique incidence of laser light increases the azimuth angle. When the Littrow incidence structure is not satisfied, the laser will increase the azimuth angle after oblique incidence. According to the generalised one-dimensional grating equation, a more general conclusion in the presence of azimuth angle is deduced, which provides a theoretical basis for the subsequent improvement of the device.
Accuracy analysis of a three-dimensional angle measurement sensor based on dual PSDs
Zhao Wenhe, Bai Yangyang, Wang Jinkai, Zhang Lizhong
2024, 53(2): 20230543. doi: 10.3788/IRLA20230543
[Abstract](25) [FullText HTML] (9) [PDF 3837KB](12)
  Objective  In systems such as airborne photoelectric turntables and multi degree of freedom swing tables, three-dimensional angle measurement is often required. The methods of angle measurement are divided into contact measurement and non-contact measurement, and different measurement methods need to be selected based on actual application scenarios. For some flexible supports, parallel support platforms, and uncertain rotation axes between the moving object and the base, non-contact measurement methods need to be considered. At present, the common non-contact three-axis angle measurement schemes are complex and occupy a large space, which cannot meet the volume and weight requirements of airborne and spaceborne payload. Therefore, it is necessary to develop a non-contact three-axis angle measurement method with a simple structure and small footprint to meet the needs of different usage environments. Therefore, a non-contact three-dimensional angle measurement system based on two position sensitive detectors (PSD) has been proposed.   Methods  A three-axis angle measurement system based on dual PSD has been established. The system mainly consists of two parts of an autocollimation measurement unit and a double-sided reflection wedge (Fig.2). The autocollimation measurement unit includes a light source, PSD1, PSD2, autocollimation lens, and subsequent processing circuits. The light beam emitted by the light source converges into parallel light through a collimating lens. And PSD1 and PSD2 receive the light spot converged by the reflected beam and perform signal processing calculations through a processing circuit. The double-sided reflective wedge is designed with a semi-reflective and semi-transparent front surface and a fully reflective rear surface. Its function is to disperse the incident collimated parallel light into two beams and reflect them back into the self-collimating lens, which converges onto the target surfaces of two PSDs to form a light spot. According to the principle of angle measurement, the calibration method of two PSDs is designed to compensate for welding errors, and the FIR filtering algorithm is used to filter the simulated collected signal to improve accuracy.   Results and Discussions   A three-axis angle measurement system based on dual PSD has been designed, and a calibration experimental system (Fig.5) has been established to calibrate the relative position relationship between two PSDs. The welding error of the relative positions of the two PSDs is compensated through the rotation matrix and translation matrix, and the compensation result is great. A 34th-order FIR filter was designed and simulated, and the experimental results show that the designed filter has a good filtering effect on the actual collected noise signals. The filter is applied to the actual processing MCU for experiments, and the phase frequency response characteristics of the selected filter are analyzed. The test results show that the response bandwidth of the filter is 1.31 kHz, which can effectively filter out high-frequency noise signals in the analog voltage signal. The angle measurement experimental system (Fig.13) has been established, and the three-axis angle measurement function of the system has been verified. The system also has high accuracy.   Conclusions  A non-contact three-axis angle measurement system based on dual PSD is designed. This system has advantages such as simple structure, small size, high accuracy, large measurement range, high bandwidth, non-contact, and insensitivity to axial translation. The rotation matrix, translation matrix, and the designed 34th-order FIR filter obtained from calibrating two PSDs are coded and written into the STM32F4 series microcontroller, and the filter delay is approximately 525 μs, which is within an acceptable range. The processing circuit and selected devices designed according to the actual requirements of the project have been experimentally verified. Within a measurement range of ± 2°, the accuracy of yaw angle measurement reaches 0.006°, pitch angle measurement accuracy reaches 0.009°, and roll angle measurement accuracy reaches 0.021°. The overall autocollimation measurement unit weighs 230 g and has a size of 50 mm × 50 mm × 50 mm square box. The response frequency of the measurement system can reach 1.15 kHz. This system can measure the three-axis angle in real-time at high speed with high accuracy and small volume, and is suitable for various engineering applications, providing stable and high-speed three-axis angle measurement solutions for airborne, spaceborne, and other conditions.
Calibration method of fisheye camera for high-precision collimation measurement
Xiong Kun, He Xuran, Wang Chunxi, Li Jiabin, Yang Changhao
2024, 53(2): 20230549. doi: 10.3788/IRLA20230549
[Abstract](19) [FullText HTML] (6) [PDF 1793KB](6)
  Objective  Collimation measurement is one of the most widely used precision angle measurement and attitude measurement methods. By imaging the known reference target at infinity, the accurate angular relationship between the measured object and the reference target can be obtained. The measurement results have the advantages of high accuracy and high repeatability. Photoelectric autocollimator, electronic total station, theodolite and other measuring and calibration instruments all take collimation measurement as their main measurement principle. Due to the limitation of the calibration accuracy of the large-field-of-view and high-distortion optical system, the camera field of view used in precision collimation measurement is usually small, so there are great limitations in the application of large range angle measurement. Fisheye camera has the advantages of large field of view, small volume and light weight, so it should have a broad development prospect in the field of measurement and calibration. However, due to the large field of view and large distortion of the fisheye camera, there is a complex nonlinearity in the camera imaging process, and the asymmetry in lens processing has a more severe impact on the imaging model parameters. For this reason, a fisheye camera calibration method for high precision collimation measurement is proposed in this paper.   Methods  A two-step fisheye camera calibration method for collimation measurement is proposed in this paper, which includes radial rough calibration based on interpolation and fine calibration based on grid compensation. This method uses interpolation instead of constructing camera model, which can effectively avoid the system error caused by inaccurate model and unreasonable parameter setting, and can restrain the asymmetry of lens processing and the deviation caused by optical system adjustment to a certain extent. Different from the commonly used performance indicators such as peak signal-to-noise ratio (PSNR) and structural similarity (SSIM), the mean reprojection error (MRE) selected in this paper can more effectively measure the camera calibration results under the condition of collimation measurement.   Results and Discussions   According to the classical model of fisheye camera, four different virtual fisheye cameras are constructed for simulation experiments (Tab.1). The simulation result shows that the calibration effect of this method on the four virtual fisheye camera models is better than the calibration method proposed recently (Fig.8), and the calibration uncertainty can be increased by 82.63% compared with the traditional method. Then, a fisheye camera calibration prototype based on embedded platform is designed (Tab.2). The calibration experimental results of the prototype shows that the proposed method can effectively calibrate the real fisheye camera for collimation measurement (Fig.10). After the calibration method is applied to the real prototype built in this paper, the uncertainty of the solution of the incident vector of the prototype can be raised to arcseconds (Fig.11).   Conclusions  A fisheye camera calibration method for high-precision collimation measurement is proposed. In the method, calibration process of fisheye camera for collimation measurement is divided into two parts of radial calibration and grid calibration. Firstly, two kinds of calibration sample points are collected with the help of high-precision turntable and collimator. Then the rough construction of the imaging model is completed by radial calibration. Finally, grid calibration is used to eliminate the error caused by the non-coincidence of rotation axis and optical axis in radial calibration, and further improve the calibration accuracy. Through simulation comparison experiments and prototype verification experiments, it is proved that this method has high calibration accuracy. Moreover, this method can be applied to the high-precision calibration of all kinds of real fisheye cameras for collimation measurement, and can provide technical support for the development of collimation measurement in the future.
Research on phase unwrapping technology based on improved U-Net network
Xu Ruishu, Luo Xiaonan, Shen Yaoqiong, Guo Chuangwei, Zhang Wentao, Guan Yuqing, Fu Yunxia, Lei Lihua
2024, 53(2): 20230564. doi: 10.3788/IRLA20230564
[Abstract](17) [FullText HTML] (7) [PDF 4321KB](5)
  Objective  Objective Phase Measurement Deflectometry (PMD) is widely employed in free-form surface transmission wavefront detection due to its simplicity, high accuracy, and broad detection range. Achieving high-precision phase acquisition is a critical step in the measurement and detection process. The phase unwrapping task, crucial in optics, plays a pivotal role in optical interferometry, magnetic resonance imaging, fringe projection profilometry (FPP), and other fields [1-4]. The challenge lies in recovering a continuously varying true phase signal from the observed wrapped phase signal within the range of [−π, π). While the ideal phase unrolling involves adding or subtracting 2π at each pixel based on the phase difference between adjacent pixels, practical applications face challenges such as noise and phase discontinuity, leading to poles in the wrapped phase [5]. These poles result in accumulated computational errors during the unwrapping process, causing phase unwrapping failures. Various methods are employed to unwrap and obtain the real phase distribution. To address these challenges, this paper proposes a phase unwrapping algorithm based on an improved U-Net network.  Methods  During the model training process, a composite loss function is defined to train the network based on the specific problem of spatial phase unwrapping. To address these challenges, this paper proposes a phase unwrapping algorithm based on an improved U-Net network. This algorithm utilizes U-Net as the basic network, integrates the CBiLSTM module for modeling time series, introduces an attention mechanism for enhanced generalization, and explores optimized loss functions. The proposed network model is validated through simulated and real datasets, showcasing its outstanding performance under noise, discontinuity, and aliasing conditions.The introduction of the attention mechanism enables better capture of global spatial relationships, while CBiLSTM effectively captures and stores long-term dependencies through memory unit structures. Memory units selectively remember and forget parts of the input signal information, enhancing their ability to handle long sequence data modeling tasks. The paper defines a composite loss function tailored to the spatial phase unwrapping problem during the model training process.Comparative experiments between the proposed network and classic models, such as U-Net [20], Res-UNet [21], and methods by Wang [13] and Perera et al. [19], demonstrate the robustness of the proposed network under severe noise and discontinuities. Additionally, it showcases computational efficiency in performing spatial phase unwrapping tasks.  Results and Discussions  Fig.10 shows the comparison between the predicted absolute phase and the real phase output by the wrapped phase after training the network model proposed in this article. Through the construction of the encoder-decoder model, the introduction of the CBiLSTM module and the attention mechanism module, and the composite The definition of the loss function, after comparing with other models, verifies the improvement in accuracy and reduction in training of the network model proposed in this article in the three situations mentioned above. Through simulation experiments and verification, by enhancing the deep learning model's ability to pay attention to key phase information, the network model proposed in this article can improve the accuracy and robustness of phase unwrapping, and promote further development in fields such as optical measurement and phase imaging.  Conclusions  This paper addresses the challenge of wrapped phase unwrapping by introducing a novel convolutional architecture framed as a regression problem. The proposed network incorporates several enhancements within the encoder-decoder framework, notably featuring a CBiLSTM module and a soft attention mechanism. Comparative analyses with existing phase unwrapping methods demonstrate the network's remarkable performance in achieving precise phase unwrapping, even in severe noise, discontinuities, and aliasing. Notably, the network showcases exceptional unwrapping capabilities without necessitating extensive training on large datasets. Moreover, it exhibits significantly reduced computational time, rendering it well-suited for tasks requiring accuracy and expeditious phase unwrapping.Validation experiments conducted on real laboratory datasets further affirm the outstanding performance of the proposed network. The introduced model empowers phase unwrapping tasks under challenging conditions, such as severe noise, discontinuities, and aliasing, surpassing the limitations of traditional methods. Comparative assessments with other deep learning models reveal a normalized root mean square error (NRMSE) as low as 0.75%. The advancement in unwrapped phase technology holds substantial significance for optical free-form surface detection, contributing to enhanced measurement accuracy, precise control of optical parameters, optimization of optical design, and quality assurance in optical manufacturing and detection processes.
Research on onboard radiation calibration scheme based on pixe-level adaptive gain imaging system
Li Ze, Wei Jun, Huang Xiaoxian, Tang Yuyu
2024, 53(2): 20230561. doi: 10.3788/IRLA20230561
[Abstract](6) [FullText HTML] (1) [PDF 4215KB](2)
  Objective  The pixel-level adaptive gain imaging system can achieve a large dynamic range of remote sensing imaging requirements while ensuring high signal-to-noise ratio by integrating four different volume integrating capacitors in the electronic link of each pixel. During prelaunch testing of this imaging system, due to the large dynamic range of Ultra Low Gain (ULG) and the limited energy of the laboratory integrating sphere, the output characteristics of the second half range of ULG can only be indirectly calibrated by recurrence of the proportional coefficient with Low Gain (LG). Excessive reflected energy from the onboard solar scaler can lead to saturation of High Gain (HG) and Medium Gain (MG) outputs, which can only be inferred through proportional coefficients and cannot be directly calibrated. A scheme for measuring the onboard proportion coefficient between adjacent gains is proposed. The test images are classified using outputs of different gains as features to obtain the output of different imaging targets, and the outputs of different targets are used as multiple calibration energy levels. The proportion coefficient of adjacent gains is obtained by linear fitting of adjacent gain outputs using the least squares method. This scheme verifies the laboratory proportionality coefficient and solves the problem of onboard radiance calibration of HG and MG.  Methods  The onboard gain ratio measurement scheme of the pixel-level adaptive gain imaging system proposed in this article mainly utilizes the incoming pupil radiance received by the detector to provide appropriate energy levels for adjacent two gain levels, in order to calculate the conversion ratio coefficient between them. During the calibration process, the type of imaging target is not taken into account, that is, the ground, clouds, etc. which are considered as energy levels. Therefore, atmospheric correction and cloud removal are not required, and it is not affected by weather. This scheme can be used to measure adjacent gain ratio coefficients on both sunny and cloudy days. The specific solution is to determine the two level gain ratio coefficients that are currently suitable for measurement through the adaptive gain imaging results of different channels; Then, the fourth gear gain output after removing stripe noise is used as the imaging feature of a single pixel to cluster the imaging images; By using different categories of image regions with adjacent gains after clustering as energy points to fit the linear relationship between adjacent gains, the ratio coefficient of adjacent gains for the current channel can be obtained. And further use the gain ratio coefficient to calculate the onboard radiation calibration coefficients for the four gains.  Results and Discussions  The removal effect of stripe noise in the imaging process of outfield experiments and the complexity of ground targets during outfield imaging may have a certain impact on the measurement accuracy of the gain to gain ratio coefficient. The proportion coefficient calculated using the method proposed in this article is used to invert the higher gain image from the lower gain image in the adjacent two levels. Compared with the actual higher gain image, the normalized mean square error is mostly less than 0.01, the structural correlation coefficient of the two images is basically around 90%, and the data correlation coefficient reaches 90%. Prove that this method has high accuracy in determining the correlation coefficient between adjacent two gain ratios, and it is highly feasible to use it for recursive calculation of high gain radiometric calibration coefficients in satellite radiometric calibration.  Conclusions  Through field imaging experiments, it has been verified that the ratio coefficient of adjacent gains measured by this method can effectively complete the task of inverting high gain images from low gain images. Therefore, it is highly feasible to use this method to obtain the other three gain radiometric calibration coefficients from the radiometric calibration coefficients of ULG during onboard radiometric calibration. This provides a certain idea for the onboard radiation calibration of large dynamic range imaging systems with multiple gains.
Test and analysis of vibration characters of damper in EO platform of UAV
Wang Zichen, Wang Donghe, Zhu Wei
2024, 53(1): 20230432. doi: 10.3788/IRLA20230432
[Abstract](33) [FullText HTML] (8) [PDF 1723KB](10)
  Objective  Absorber is a very commonly used component in UAV OE platform, because it can effectively absorb the vibration caused by the UAV, thereby enabling the photoelectric platform to obtain more stable and clear image or video. In the recent years, many scholars focus their attention on many kinds of damper in order to further improve the imaging quality of the OE platform. Non-angular displacement damper is designed for optimizing the performance of platform payload, but its practicality is limited by large volume and heavy weight. So traditional damping absorber is still the most widely used damper in OE platform at present. A lot of research have been conducted about the characters of damping absorber, but quantitative analysis to address the impact of vibration on optoelectronic platforms and how to optimize the installation layout of OE platform with damping absorber has not been carried out.   Methods  According to the characters of airborne OE payload, this paper analyzes and tests the angular characteristics of damping absorber that is related to the performance of airborne payload. First, the self-character of damper of EO payload is introduced. Second, the overall performance model of EO payload are established. Meanwhile, the modeling of motion characteristics is established and the factors that may affect the ability of airborne payload are analyzed during the installation layout of damping absorber. Many factors, such as stiffness, damping coefficient, installment-center-distance and the mass of payload, are promoted which determine the results of angular and linear displacement.   Results and Discussions  The results of simulation analysis show that the displacement of damper mainly occurs in the vertical direction instead of horizontal direction, and the displacement in horizontal direction is only 2% in the vertical direction within the effective stroke of damper when the distance of rotation center R is set as 200 mm, 250 mm and 300 mm. The portable test system is mounted using a vibration table, an UAV OE platform with four damping absorbers and an auto-collimator. All the dampers, with the same stiffness and damping coefficient, are fixed between vibration table and UAV OE platform, and auto-collimator with target is used for testing the impact of vibration on payload imaging quality. Four images are captured by OE payload when the installment-center-distance is set as 350 mm, 300 mm, 200 mm and 100 mm respectively, and the vibration table is under 5 Hz low-frequency disturbances. It is not difficult to find that with the increase of installment-center-distance, the image quality decreases significantly. So, we should increase installation spacing of diagonal dampers as much as possible in application.   Conclusions  According to the characters of OE payload, we researched the characteristics, motion modeling, and testing applications of UAV OE platform and its related damping absorber deeply. On the basis of introducing the working principles of damping absorber and OE platform, a motion characteristic model of the platform with damping absorber is established. At the same time, not only the theoretical model is simulated, but also the test results are conducted in laboratory. All the results indicate that the model can reflect the impact of damping absorber on the overall performance of UAV optoelectronic platform basically, and the results of our research in this paper are very helpful in optimizing the design, installation, and practical engineering applications of UAV platform.
Research on fine characterization technology of key parameters of line width of Si/SiO2 multilayer film
Chu Xiaoyao, Shen Yaoqiong, Liu Liqin, Zou Wenzhe, Guan Yuqing, Guo Chuangwei, Zhang Yujie, Liang Lijie, Kong Ming, Lei Lihua
2024, 53(1): 20230475. doi: 10.3788/IRLA20230475
[Abstract](27) [FullText HTML] (6) [PDF 3488KB](4)
  Objective  As the key parameters of line width, line edge roughness (LER) and line width roughness (LWR) are important indicators of the quality of line width standard samples. The accuracy of LER and LWR is important for characterizing the reliability and uniformity of line width standard materials. Inspection is very important. Through the measurement and characterization of LER and LWR, the quality label technology level of line width standard samples can be effectively evaluated. Due to the problem of magnification in the measurement method of SEM, the measurement and characterization of LER and LWR have trays. Therefore, before using SEM to measure the line width, it is necessary to adjust the magnification of SEM with standard substances in advance.   Methods  With the self-traceable grating reference material as the standard of mass transmission (Fig.2), SEM is used to scan the self-traceable grating reference material, and the grating period measurement value of the self-traceable grating is obtained (Fig.3). It is compared with the actual grating period value, and the SEM calibration factor is obtained to realize the direct traceability and magnification calibration of the scanning electron microscope. The calibrated SEM is used to measure the different values ​​of the multilayer film line width standard samples in different areas and different magnifications. The image processing technology is used to determine the position of the line edge and the average line edge based on the least squares fitting method. The root mean square roughness of the amplitude quantization parameter is calculated for LER and LWR (Fig.4).   Results and Discussions   The calibration of different magnifications of SEM is realized, and the calibration factors under different magnifications are obtained, which ensures the accuracy and traceability of the measurement results and shortens the traceability chain. The measurement results of line widths of different sizes are basically the same at different positions and different magnifications (Tab.2, Fig.8), the fluctuation range of line edge roughness is relatively small, the measured values ​​are relatively consistent, and the change of line width is small (Tab.3, Fig.9); It shows that the edge of the line width sample is relatively smooth, the line width distribution is relatively uniform, and has good uniformity and consistency, which shows that the Si/SiO2 multilayer film deposition technology has the advantages in controlling the line width size and edge characteristics.   Conclusions  The SEM value traceability and magnification calibration method based on the self-traceable grating standard material shortens the traceability chain, reduces the traceability error introduced in the process of value traceability, improves the accuracy and reliability of SEM measurement, and provides a possibility for the flattening of the value transfer gradually. Through the measurement and analysis of line edge roughness and line width roughness, accurate characterization of line width and edge characteristics is achieved, and metrological support is provided for high-precision nanoscale measurement and microelectronics manufacturing fields.
Research on jitter compensation algorithm in spectral confocal thickness measurement system
Li Chunyan, Li Danlin, Liu Jihong, Liu Chang, Li Ke, Jiang Jiewei
2024, 53(1): 20230444. doi: 10.3788/IRLA20230444
[Abstract](30) [FullText HTML] (7) [PDF 4377KB](9)
In order to obtain multi-point data of the sample, the spectral confocal displacement sensing system will produce jitter effect when moving the measurement, causing the drift of the measurement data. Based on the realized spectral confocal thickness measurement system, the effect of jitter is studied and the jitter compensation algorithm is explored; Firstly, based on the spectral confocal thickness measurement model and the presence of jitter when the probe is tilted to a certain extent relative to the optical axis, the relational model of the effect of jitter on the thickness measurement is deduced, and the thicknesses of four kinds of samples with different degrees of random jitter are analyzed by Monte Carlo simulation method. The analyzed results are compared with the Monte Carlo simulation results to verify the correctness of the expression of the thickness probability density function. The results show that the jitter effect leads to a degradation of the measurement performance, especially when the sample thickness is large; In the case of a large standard deviation of jitter, the measurement of thinner samples has a better anti-jitter performance. Then, in order to compensate the effect of jitter on the measurement results, it is proposed to use Savitzky-Golay filtering and Gaussian fitting to realize the filtering and the extraction of the peak wavelength of the spectral signal, and the jitter error compensation algorithm is established; Finally, experimental measurements were conducted on a sample with a thickness of (1.0±0.1) mm, and the average thickness was measured to be 1.064 0 mm. The compensated relative standard deviation was 0.29%, verifying the effectiveness of the jitter compensation algorithm. This research has some guiding significance to improve the system measurement stability and measurement accuracy.  Objective  With the development of miniature integrated optical instruments such as optical communications and optical sensing, the use of transparent materials is becoming more and more stringent. Highly accurate thickness measurement parameters help guide their precise application and control the performance of related ultra-precision optical instruments, making accurate thickness inspection necessary. Spectral confocal method uses a broad-spectrum light source to irradiate the surface of the object, uses the principle of optical dispersion to make the dispersive objective lens produce axial chromatic aberration, establishes the correspondence between the dispersive distance and the wavelength, and uses a spectrometer to detect the peak wavelength of the spectrum that is focused on the surface of the object and reflected back to get the accurate axial position or micro-displacement data. Such an approach allows to break through the diffraction limit of ordinary optical microscopes. It achieves ultra-high ranging resolution on the nanometer scale and has wide adaptability to environments and materials. When measuring the thickness of transparent materials using the spectral confocal method, the jitter effect alters the refractive properties of the beam entering the sample and random noise is present in the received spectral response curve reflected from the sample surface, which leads to drift of the measurement data. On this basis, the relationship model of the effect of jitter on the spectral confocal thickness measurement is firstly derived in this paper, and the distribution of the thickness probability density function of the sample under different degrees of random jitter is simulated and analyzed by Monte Carlo method. In order to compensate for the effect of jitter on the measurement results, it was proposed in this paper to use Savitzky-Golay filtering and Gaussian fitting to extract the peak wavelength of the spectral signal, and a jitter compensation algorithm was established. Finally, the stability of the measurement results is improved by experimental measurements, and the effectiveness of the algorithm is verified.   Methods  The effect of jitter on thickness measurement of transparent materials is studied in this paper. First, the thickness measurement models of the probe with respect to the optical axis were derived when the probe was not tilted (Fig.2) and tilted (Fig.3), and the influence of jitter on the thickness measurement results was characterized by the optical axis tilt, and simulation analysis was carried out (Fig.4). Then, by comparing thickness measurement errors and jitter standard deviation at different wavelengths (Fig.5), after comparative analysis of various algorithms, spectral noise was filtered by Savitzky-Golay filtering algorithm (Fig.7-8), and peak wavelength of spectral signal was extracted by Gaussian fitting algorithm (Tab.1). An optimized jitter compensation algorithm is constructed. Finally, the validity of Savitzky-Golay filtering algorithm and Gaussian fitting algorithm for jitter compensation in spectral confocal thickness measurement is verified (Tab.2).   Results and Discussions   The thickness of the sample under static conditions depends only on the focusing wavelength, the angle of incidence and the refractive index of the transparent material. Random jitter angle is the main source of thickness measurement error, and the thickness measurement error caused by sensor probe jitter should not be neglected. By analyzing the effect of random jitter angle on the measurement error, a jitter compensation mechanism is established to reduce the measurement error. Thickness measurement data is non-central cardinality distribution, the jitter effect will lead to the measurement performance degradation, especially in the case of larger sample thickness; In the case of larger jitter standard deviation, thinner samples have better anti-jitter performance. The peak wavelength of the spectral signal is extracted by S-G filtering and Gaussian fitting, which can reduce the error caused by the mechanical vibration of the probe and improve the measurement stability of the confocal spectroscopy measurement system.   Conclusions  In this paper, based on the spectral confocal method to realize the thickness measurement of transparent materials, the jitter effect generated by the movement will make the measurement data drift, and the influence of the jitter effect on the thickness measurement is systematically studied. Firstly, the relationship model of the effect of jitter on thickness measurement is established based on the principle of spectral confocal thickness measurement system, theoretical derivation is carried out, and MC simulation is used for simulation verification. Secondly, the thickness PDF and MC simulation results are compared and analyzed to verify the correctness of the thickness PDF expression. The results show that the jitter effect leads to a degradation of the measurement performance, especially when the sample thickness is large; In the case of a large standard deviation of jitter, the measurement of thinner samples has a better anti-jitter performance. In order to correct or compensate the effect of jitter on the measurement results, it is proposed to use S-G filtering and Gaussian fitting to realize the random noise filtering and the extraction of the peak wavelength of the spectral signal, and the jitter error compensation algorithm is modeled. Finally, experimental measurements were conducted on a sample with a thickness of (1.0±0.1) mm. Under stable conditions, the average thickness was measured to be 1.064 0 mm. The relative standard deviation of the moving measurement results was reduced from 1.86% before compensation to 0.29% after compensation, verifying the effectiveness of the jitter error compensation algorithm proposed in this paper and proposing improvement measures. The results of this paper have certain guiding significance for optimizing the system structure and further improving the performance of the system, and have certain advancing effect on the practical application of the spectral confocal displacement sensing system for stable measurement.
High dynamic range surface measurement method based on adaptive multi-exposure fusion
Lei Jingfa, Xie Haoran, Li Yongling, Wu Dong, Zhang Miao, Zhao Ruhai
2024, 53(1): 20230370. doi: 10.3788/IRLA20230370
[Abstract](40) [FullText HTML] (5) [PDF 5173KB](10)
  Objective   For low dynamic range surface objects, a single exposure can provide sufficient exposure, but for high dynamic range surface objects, it is difficult to obtain high-quality fringe patterns with a single exposure, the multi-exposure fusion technology fuses the fringe pattern through multiple exposures, which can effectively improve the definition of the fringe pattern, thereby improving the accuracy of phase measurement. The traditional multi-exposure fusion technology needs to manually set the exposure time, which has problems such as low efficiency and poor exposure accuracy, in this paper, the method of adaptive exposure is used to obtain the exposure time, which avoids the disadvantages of manual exposure. Although the fringe image fused by traditional multi-exposure fusion technology has removed overexposure points, the overall quality of the fringe image is still not high, therefore, this paper improves the fusion process of multi-exposure images, a fringe map with better image quality is obtained.   Methods   Firstly, the images taken under the initial exposure time are analyzed by histogram, the areas with different reflectance on the surface of the measured object are divided into several groups, the optimal exposure time of each group is calculated respectively; on this basis, the images of projected white light and projected stripes were taken under the optimal exposure time corresponding to different groups, after removing the high gray value area exceeding the set threshold in the image, then making the image collected when projecting white light into a mask image, and multiplied with the image acquired when the fringes were projected at the same exposure time, perform brightness compression and fusion on multi-group multiplied images; finally, improve the contrast and clarity of the fringe image generated after fusion through the CLAHE algorithm, thereby performing stripe unwrapping and point cloud reconstruction.   Results and Discussions   The adaptive exposure used in this paper is more efficient and accurate compared to manual exposure (Fig.6), for the three high dynamic range surface objects of the U-card, the Connection Bock and the Disc, the fringe image fused by the method in this paper has no overly bright and overly dark areas, and the overall quality of the fringe pattern is good (Fig.7, Fig.8, Fig.9), there is no obvious loss in the point cloud\image after 3D reconstruction (Fig.10, Fig.11, Fig.12), the number of point clouds measured by the method in this paper is similar to the number of three-dimensional point clouds measured by the spray imaging agent method, the reconstruction rate has reached more than 99.5% (Fig.13), the measured absolute error and relative error of the standard block step height are only 0.062 mm and 0.69% (Tab.2).   Conclusions   Aiming at the failure of 3D contour detection of high dynamic range surface objects, this paper proposes an improved multi-exposure fusion method, replacing manually setting exposure with adaptive exposure, at the same time, in the process of image fusion, the contrast and clarity of the fringe image are improved by setting the threshold, brightness compression, and using the CLAHE algorithm. Experimental results show that adaptive exposure is more efficient and accurate than manually setting exposure, the point cloud reconstruction rate of different high dynamic range surface objects is above 99.75%, the measured absolute error and relative error of the standard block step height are only 0.062 mm and 0.69%, effectively solved the problem of missing 3D point clouds for detecting high dynamic range surface objects, improved measurement efficiency and accuracy of 3D profile measurement.
Comparison and analysis of the overall architecture of foreign EO targeting pod detection system
Li Jian, Zhang Dayong
2024, 53(1): 20230353. doi: 10.3788/IRLA20230353
[Abstract](35) [FullText HTML] (7) [PDF 2228KB](15)
  Significance   The technical characteristics and development process of four generations of EO (electro-optical) targeting pod are compared. The design focus and key points of each generation are summarized. Focusing on the overall design of the detection system of AN/AAQ-33 Sniper XR ATP pod and AN/ASQ-228 ATFLIR pod, as well as the newly emerging fourth generation products such as Litening 5 and Talios pods, this paper provides a reference for the development of the new generation EO targeting pod detection system.   Progress  Firstly, the ratio of optical aperture to pod diameter (ROP) is defined as a standard for measuring the integration level of optical machinery and servo control system. The higher the ROP is, the higher the system integration degree is.   Secondly, the optical system of ATP and ATFLIR is analyzed, which are regarded as the typical 3rd generation targeting pod, both adopt the series common optical path architecture of front telescope system and servo frame platform are placed at the head of the pod. And the compressed parallel beams are introduced into the beam splitter and rear detection/laser emission system which are placed at the middle of the pod through the optical hinge and fast steering mirror (FSM). The two pods' small field of view (sFOV) is about 1.5°×1.5°, and the wavelength is 0.7-0.9 μm & 3.7-4.8 μm, and their modulation of transfer function (MTF) are close to diffraction limit. A refractive front telescope system like ATP with φ150 mm common optical path is forward designed, and the result verifies the optical system considering the servo frame platform can be installed into a pod of φ305 mm diameter, and the ROP is 0.492; A off-axis three mirror astigmatism (TMA) front telescope system like ATFLIR with φ150 mm common optical path is forward designed, and the result verifies the optical system can be installed into a pod of φ330 mm diameter, and the ROP is 0.455 (Fig.4, Fig.7).   Finally, as a comparison, the optical parameters of Litening 5 and Talios (Fig.9-10) are introduced, which are regarded as the fourth generation targeting pods. The detection systems adopt a parallel common cabin layout, with all the optical payload and servo frame platform installed inside a sphere with φ406 mm diameter, and their largest optical apertures are still φ150 mm. The ROP of Litening 5 and Talios is 0.37 and 0.38, much lower than ATP and ATFLIR, indicating low integration levels. Litening 5 pod addes a shortwave infrared imaging band, which has high fog penetration ability; Talios pod addes the step-and-stare scan imaging ability; Both of them add visible light color imaging function to improve detection and recognition probability. However, based on the optical design analysis, ATP and ATFLIR can easily modify their optical systems to achieve these functions, indicating the two pods have strong vitality due to their forward-looking overall architectures, and the improvement of ATFLIR optics is much easier than that of ATP.   Conclusions and Prospects  In the future, the targeting pod needs to integrate functions such as air-to-air detection, laser communication, and directional infrared countermeasures (DIRCM), and must have high functional density. The series layout architecture using a pure reflection common path front telescope system, optical hinges, FSM, rear detection/laser emission system has strong scalability and expansibility.
Field prototype for rapid classification of suspended particles in water based on polarized light scattering and fluorescence measurement
Xiong Zhihang, Mai Haoji, Huang Zhuangfan, Li Jingteng, Sun Peitao, Wang Jialin, Xie Yongtao, He Zixi, Zeng Yaguang, Wang Hongjian, Guo Zhiming, Liao Ran, Ma Hui
2023, 52(9): 20230030. doi: 10.3788/IRLA20230030
[Abstract](99) [FullText HTML] (19) [PDF 2621KB](21)
  Objective  Suspended particles in water include solid or liquid particles, such as sediment, microplastics, and microalgae. Accurate monitoring of their categories and concentration is of great scientific and practical significance for studying and protecting aquatic ecosystems. Various optical instruments have been developed to probe suspended particles in water, which can be divided into two categories based on the measurement methods. One category measures the overall characteristics of all particles in a body of water, while the other measures individual particles. Water Quality Analyzer (QWA) provide estimates of particle concentration and size distribution, chlorophyll-a concentration, and other water quality parameters. However, QWA are limited in their ability to accurately identify the categories of suspended particles in water. Underwater flow cytometry enables the characterization of various categories of particles by breaking up a water sample into individual particles that are then to be measured. However, this technique is expensive and requires complex sample pretreatment, which limits its application. Therefore, it is needed to develop a prototype for field detection of water samples collected in the wild, with the goal of quickly determining the categories, numbers, and proportions of suspended particles in water.  Methods  Suspended Particle Classifier (SPC) has been developed in this paper and its diagram is depicted (Fig.1). The SPC employs a 445 nm laser as the excitation source to induce chlorophyll fluorescence, and the polarization state of the laser is modulated by a polarization state generator. The SPC obtains individual particle polarized light scattering and fluorescence signals, which are combined with a Support Vector Machine (SVM) to classify particles based on their optical properties. To ensure its suitability for field use, the SPC is equipped with a drainage tube for the transportation of water samples and an industrial computer for instrument control and data analysis. Standard samples of sediments, microplastics, and microalgae are collected. Then, datasets are created to train the SVM classifier. Subsequently, SPC was deployed alongside QWA in the Yamen Waterway for 25 hours to evaluate its performance (Fig.3). The accuracy of the SPC classification was verified using data obtained from the QWA.  Results and Discussions  The SPC's classification accuracy for standard samples of sediment, microplastics, and microalgae was found to be 95.3%, 93.3%, and 97.9% (Fig.4), respectively, indicating that the classifier has good performance in classifying these particles. The average accuracy and recall rate were found to be 95.5% (Tab.1), indicating the SVM model has strong feature extraction ability. These results suggest that the SPC can accurately classify standard samples. When applied in the Yamen Waterway, the SPC was able to rapidly measure water samples collected in the field and track the changes in the number of sediments, microplastic, and microalgae in different water layers over time (Fig.5). Furthermore, the number of microalgae identified by the SPC was found to have a strong correlation with the concentration of chlorophyll-a and phycoerythrin measured by the QWA (Fig.6, Tab.2). Additionally, the so-called effective time cross-section of sediments identified by the SPC was found to have a strong correlation with the turbidity value measured by the QWA (Fig.6, Tab.2), further validating the reliability of the SPC's classification results.  Conclusions  In this study, a suspended particle classifier was developed with the aim of classifying and counting suspended particles in water samples collected in the field. The SPC probes polarized light scattering and fluorescence signals from individual suspended particles and uses SVM to classify them based on their optical properties. The classification accuracy for standard samples of sediment, microplastics, and microalgae was over 95%. To validate the SPC's classification ability for field water samples, the SPC and QWA were deployed in the Yamen Waterway for 25 hours of synchronous testing. The SPC was able to track changes in the number of sediment, microplastic, and microalgae in different water layers over time. There was a strong correlation between the SPC and QWA measurement data, indicating the high reliability of the SPC in classifying particles in field water samples. These results demonstrate that the SPC can rapidly detect and classify suspended particles in water and has the potential to be a valuable tool for exploring aquatic ecosystems.
Observation experiment on star-light deflection of star-points under high-speed mixing flow
Chen Bing, Chen Shaojie, Chen Xiao, Li Chonghui, Zheng Yong
2023, 52(9): 20220802. doi: 10.3788/IRLA20220802
[Abstract](40) [FullText HTML] (11) [PDF 3290KB](16)
  Objective  Celestial navigation is an important method of autonomous navigation. Astronomical observation of high-speed aircraft will be disturbed inevitably by the high-speed flow nearby the observation window, which causes the star maps degradation like displacement and blurring. And this will lead to a decrease in the accuracy of the stars center, which will have a direct effect on the accuracy of astronomical attitude determination. At present, most studies on the calculation and correction of star map degradation are based on computer simulation, whose results are greatly affected by the configuration of model parameters and may not be consistent with the real physical process. Therefore, it is necessary to construct the physical experimental observation conditions of the influence of high-speed flow on star-light deflection and to carry out experimental research.   Methods  A small static wind tunnel is built, which can generate a Mach 2.5/3.5 mixing layer structure in the test section. The calibrated simulated star-points on the indoor dome with a diameter of 10 m are measured through the high-speed flow, and the star centroids are extracted to collect the data of imaging displacements by the real flow. The data of star image disturbed by the flow field are obtained and compared with the computer simulation results.   Results and Discussions   The deflection by flow is greater than the estimated value of computer simulations. At the near end of the tunnel nozzle, the high-speed mixing layer makes a large star-light deflection. The mean deflection in the vertical direction of the flow field is less than 0.5″, and that in the direction of the flow field is 3.85″, and the maximum is close to 4.89″ (Fig.8). At the far end, the mean deflection in these two direction is −1.36″ and −0.49″ respectively (Fig.9). The variation of starlignt deflection at the near end is smaller and more stable than that at the far end, which is conducive for modeling correction (Fig.10).   Conclusions  A star-points observation system under the high-speed flow was constructed based on the indoor dome, and a Mach 2.5/3.5 mixed high-speed flow field was generated in the experimental observation section. The target star-points were observed from different observation positions, and the quantitative conclusion of the high-speed flow on star-points imaging disturbance was obtained for the first time by physical observation experiment.   The results show that: 1) Star-light deflection is mainly concentrated in streamwise. This result is consistent with the expectation of the theoretical analysis; 2) The star-light deflection caused by the flow field at the near end of the nozzle is larger than that at the far end, but the variation range is smaller and more stable than that at the far end, which is conducive for modeling correction; 3) The absolute value of target starlight deflection caused by high-speed mixed flow is greater than the simulation result at both near and far end of the tunnel nozzle. The current work has proved the stability and effectiveness of the experimental system, which can provide an experimental basis to form a systematic understanding of the influence of flow structure on navigation starlight acquisition by the subsequent systematic observation under different altitude angles and azimuth angles, and provide experimental data of physical observation for simulation modeling. Then, a modified model of the influence of high-speed flow fields with different structures on starlight could be established, which may provide theoretical support for the suppression of aerodynamic influence and the deflection correction of air-cooled film in the astronomical observation of hypersonic vehicles.
Research on harmonic detection pressure inversion based on Gauss/Lorentz line fitting ratio (invited)
Gao Nan, Yu Yongbo, Du Zhenhui, Li Jinyi, Meng Zhaozong, Zhang Zonghua
2023, 52(8): 20230428. doi: 10.3788/IRLA20230428
[Abstract](61) [FullText HTML] (8) [PDF 1985KB](22)
  Objective  Tuned laser absorption spectroscopy (TLAS) technology has advantages such as non-contact, anti-interference, and high sensitivity, which can be used for gas concentration, temperature, and pressure measurement. In the existing pressure detection models, limited feature points of spectral lines are mostly extracted and calculated, which can lead to problems such as susceptibility to interference in measurement results and significant measurement errors. Therefore, it is necessary to establish a new anti-interference and stable pressure detection model. To solve this problem, a mathematical model was proposed for fitting the pressure and spectral line shape function within the low and high pressure ranges based on the gas pressure measurement method of absorption line width.   Methods  Simulation research on the second harmonic absorption lines under different pressures was conducted based on the principle of spectral line broadening. In order to simulate the pressure changes by adjusting the Gauss/Lorentz halfwidth ratio, the second-order derivative signal was obtained by convolution of Gauss and Lorentz functions to simulate the second harmonic of the absorption spectral line. By establishing a mathematical model of the Gauss/Lorentz line fitting ratio and pressure, the fitting relationship between the two was obtained under ideal conditions and the influence of laser line width, white noise, and background interference. The comparative analysis on the stability of the fitting ratio with eigenvalues used to calculate pressure in existing models such as the peak width and 2f/4f amplitude under dynamic noise and background interference were conducted. Finally, the measured signal at 1 580 nm of CO2 gas was processed to verify the simulation results.   Results and Discussions   The simulation results show that under ideal conditions and the influence of laser linewidth, white noise, and background interference, there is a third-order fitting relationship between the Gauss/Lorentz line fitting ratio and pressure, and the fitting degree remains above 0.998 0 (Fig.3-6). Compared with traditional models, it has better stability under dynamic noise and background interference (Tab.1). The experimental results show that the third-order fit between the Gauss/Lorentz line fitting ratio of the actual detection spectral line and the pressure is 0.986 3 (Fig.9), slightly lower than the simulated fit of 0.998 7 (Fig.2), which is consistent with the simulation analysis results.   Conclusions  In order to establish a more effective pressure detection method, based on the principle of spectral line broadening, the pressure change is simulated using the ratio of Gauss function half width to Lorentz function half width, and the Voigt function is used to describe the absorption spectral line shape. A mathematical model was established for the pressure to fitting ratio under ideal conditions, laser linewidth, white noise, and background interference. Through simulation analysis, the relationship between pressure and fitting ratio satisfies a third-order fitting relationship, which is not only affected by laser linewidth, white noise, and background interference, but also maintain stability under dynamic noise and background interference, which exhibits advantages in pressure detection compared to traditional models. The experimental validation was carried out using CO2 absorption spectra. The curve fitting obtained from analyzing the experimental data was slightly lower, but its trend was consistent, which indicated the effectiveness of the established mathematical model. The proposed method has certain theoretical significance and practical value in pressure measurement, providing new ideas for pressure detection.
Depth range enhancement of three-dimensional profiling measurement technology based on dithering algorithms
Yang Jingwen, Zhang Zonghua, Fu Lina, Li Yanling, Gao Nan, Gao Feng
2023, 52(8): 20230059. doi: 10.3788/IRLA20230059
[Abstract](125) [FullText HTML] (26) [PDF 5622KB](62)
  Objective  With the rapid development of modern information technology, optical three-dimensional (3D) profiling measurement technology has gradually matured. Among numerous optical 3D profiling measurement technologies, due to its non-contact and high-accuracy measurement, digital fringe projection(DFP) technology is increasingly applied in the fields such as biomedical monitoring, virtual reality, and computer vision, and has broad prospects for development due to its non-contact and high measurement accuracy. However, this technology still faces some technical challenges: 1) Due to the limited depth of field of the system equipment (such as cameras and projectors), only the 3D shape of objects within a limited depth of field can be reconstructed; 2) Nonlinearity problems caused by the γ-effect of commercial projectors may affect measurement accuracy. To overcome these problems, this paper proposes a method to extend the measurement depth range, which can achieve high-accuracy measurement of multiple objects at different depths or objects with a large depth range.   Methods  The paper proposes a novel method for measuring the 3D shape of objects with a large depth range. Firstly, defocus technique is used to measure the dithering pattern in a simulated sinusoidal mode, avoiding the influence of projector non-linear errors on 3D measurement of fringe projection and increasing the measurement speed. Then, by analyzing the relationship between the degree of defocusing of the fringe and the depth (Fig.1-2), this paper analyses the relationship between fringe defocus and depth and finds that the defocus degree of fringes at different frequencies is inconsistent with the depth variation nodes. Based on this, a multi-frequency phase selection method is proposed in this paper. The optimal frequency mode determination algorithm (Fig.4) is used to select the bayer dithering algorithm and the floyd-steinberg dithering algorithm to generate dithering patterns. After comparing the phase error distribution of the fringe images within the range of 12-60 pixel in period at 25 defocus levels, the defocusing selection range of the corresponding fringe frequency is screened to determine the optimal selection of fringe frequency at different defocusing degrees. Then, in order to obtain a binary pattern with the highest quality sinusoidal structure, 8 different scanning orders are used based on the selection results of the optimal frequency mode which is to select the optimal dithering mode for the current frequency (Tab.1). Finally, the method uses the selected dithering fringe pattern within the optimal frequency range to obtain the 3D shape of the object. The proposed method can extend the measurement range of object depth by selecting multi-frequency dithering fringe and determining the optimal frequency at different defocus degrees.   Results and Discussions   This paper presents qualitative and quantitative comparison experiments between the standard sinusoidal fringe and the proposed method. In the qualitative experiment, both methods are used to reconstruct the 3D shape of an object with a depth of 22.5 cm (Fig. 9). The measurement results of the proposed method are better than those of the standard sinusoidal fringe method with complete shape, clear details and without ripple phenomenon. Moreover, in the quantitative experiment, the maximum absolute error of the proposed method is 0.033 mm (Tab.2), which is comparable to the measurement accuracy of traditional DFP technology. Therefore, the proposed method not only ensures measurement accuracy but also extends the measurement depth range, and effectively solves the problem of measuring the 3D shape of objects with a large depth in the DPF field.   Conclusions  This paper proposes a MFPS method based on dithering algorithms to solve the limited measurement depth range and nonlinearity problem of the existing DFP technology. By using defocusing dithering techniques, the impact of projector nonlinearity error is overcome. Moreover, the MFPS method is used to generate dithering fringe patterns for measurement, which extends the measurement depth range. Experimental results demonstrate that the proposed method effectively extends the measurement depth range and achieve 3D shape measurements of objects in a large depth range.
Projection chromatic aberration modeling and correction of phase measurement profilometry based on phase target
Zhang Yuzhuo, Jia Lulu, Gao Nan, Meng Zhaozong, Zhang Zonghua
2023, 52(8): 20230385. doi: 10.3788/IRLA20230385
[Abstract](96) [FullText HTML] (23) [PDF 3087KB](30)
  Objective  Due to the advantages of high precision, easy recognition and high degree of automation, the three-channel phase measurement profilometry in optical three-dimensional measurement has gained increasing attention in both scientific research and engineering applications. For three-channel phase measurement profilometry, the chromatic aberration between projector channels is the key factor affecting the measurement accuracy. Most of the existing chromatic aberration correction methods of projectors regard projectors as "reverse cameras". Therefore, the accuracy of correction results will be dependent on the imaging quality of the camera. Moreover, the existing chromatic aberration measurement and correction methods still have shortcomings, so it is significant to improve the measurement accuracy of the system. Therefore, this study carries out the research on the projection chromatic aberration modeling and correction of phase target-based phase measurement profilometry.   Methods  In this paper, the projection chromatic aberration modeling and correction method using the LCD screen with holographic projection film as the phase target is proposed (Fig.3). Firstly, the unfolded phase of LCD display fringes and projector projection fringes are calculated respectively. Next, binary fitting on display phase and projection phase are carried out. The green channel is regarded as an ideal channel, and the ideal pixel values of red and blue channels is calculated. Then the ideal pixel is substituted into the projection equation, and the ideal phases of the red and blue channels are obtained. Thus, the mathematical model of the chromatic aberration of the projector is established. Finally, the pre-compensation of projection fringes is implemented with the established chromatic aberration model(Fig.5). Then, the pre-compensated fringes are projected in three channels, so that the chromatic aberration of the projector is corrected.   Results and Discussions   The experimental results demonstrated the performance of the proposed method. The average chromatic aberration of the blue and green channels is corrected from 0.325 5 pixel to 0.106 3 pixel. The average chromatic aberration of the red and green channels is corrected from 0.365 1 pixel to 0.111 4 pixel (Fig.10). This method can effectively improve the projection quality for three-channel phase measurement profilometry. The average error of the measured step is reduced from 0.489 mm to 0.038 mm (Tab.2). The experimental results verified the effectiveness of the chromatic aberration modeling and correction method of projector. This method can improve the overall measurement accuracy of three-channel phase measurement profilometry. Compared with the existing methods, the proposed method can be calibrated to avoid the impact of camera errors and effectively shorten the calculation time. Moreover, this method can be applied to the measurement and correction of different projector chromatic aberration.   Conclusions  A phase-measurement contouring chromatic aberration modeling method using an LCD display as a phase target is designed and calibrated for study. This method eliminated the coupling error of the camera while measuring and calibrating the projector chromatic aberration, and enabled measurement of the projector chromatic aberration at global pixel points, while using mathematical modeling to model the projector chromatic aberration in a chromatic way to shorten the calculation time. By measuring the 3D shape of the actual object for accuracy comparison experiments and comparing the accuracy error before and after correcting the chromatic aberration of CP270 projector and PRO4500 projector, it can be concluded that the projection chromatic aberration modeling and correction study based on phase target proposed in this paper can better improve the projection quality in phase contour measurement and enhance the measurement accuracy of commercial projectors with poor accuracy. For the projectors with low accuracy, the method of correcting chromatic aberration in this paper can greatly improve the measurement accuracy of projectors. For the projectors with high accuracy, the proposed projector chromatic aberration modeling and correction method can further improve the measurement accuracy.
3D surface shape measurement of high dynamic range object based on monochrome fringe projection
Wang Zhangying, Zhang Ningning, Gao Nan, Li Kui, Meng Zhaozong, Zhang Zonghua
2023, 52(8): 20230327. doi: 10.3788/IRLA20230327
[Abstract](97) [FullText HTML] (19) [PDF 3004KB](48)
  Objective  Due to its high speed and high precision, fringe projection profilometry has been widely used in many fields, such as automatic online inspection of mechanical parts, automobile manufacturing, cultural heritage protection. However, traditional fringe projection uses a single exposure time or a single projection intensity to measure objects with high dynamic range (HDR). Overexposure will occur in areas with large reflectance, which exceeds the maximum brightness range of the camera sensor, resulting in the failure to obtain true intensity and accurate three-dimensional (3D) data. To solve this issue, this paper proposes a HDR object surface 3D measurement method utilizing the different color channel responses of a color camera based on monochrome fringe projection.   Methods  In this paper, a 3D measurement method of HDR object surface based on monochrome fringe projection is proposed. In this method, the blue fringe patterns are projected onto the surface of the measured object, and the color camera captures the color image from another perspective. The two fringe patterns corresponding to the blue and green channels from the captured fringe images are separated. Mask image of blue and green channels are generated by selecting a group of pixels with unsaturated and maximum modulation from the blue-green channel fringe patterns. The HDR image is synthesized by the mask images of blue and green channels and fringe patterns of blue and green channels (Fig.3). Then, phase calculation methods and system calibration are applied to achieve 3D measurement of objects with high dynamic range.   Results and Discussions   To demonstrate the effectiveness of the proposed method, a metal flat plates and a metal spherical part with HDR surface were tested. Comparative experiments were conducted between the separated blue channel fringe patterns and the synthesized HDR images to verify the effectiveness of the proposed HDR method (Fig.8, Fig.10). The proposed method can provide accurate 3D measurement results without measurement errors caused by pixel saturation. To quantitatively evaluate the accuracy of the method proposed, an artificial standard step surface were measured by Zhang's method and the proposed HDR images (Fig.12). 3D data of the step surface measured by the coordinate measuring machine (CMM) can be used as the ground truth. The difference between the measured data and the ground truth are shown (Tab.1). It can be seen that the accuracy of the proposed method is slightly higher than Zhang's method. Compared with the existing HDR methods, the proposed method has the advantage of fewer images and without additional hardware facilities.   Conclusions  The 3D measurement technique for HDR object surface based on monochrome fringe projection is proposed by utilizing the different color channel responses of a color camera. The blue fringe patterns are projected onto the surface of the tested object, and captured by the color CCD camera. Fringe patterns corresponding to the blue and green channels are separated from the captured color fringe patterns. A group of pixels with unsaturated and maximum modulation from the blue and green channel fringe patterns are selected to generate mask images of the blue and green channels. The HDR image is synthesize by the mask images of the blue and green channels and fringe patterns of blue and green channels. Then the absolute phase is obtained by the phase calculation method, and the HDR object is measured by the calibrated system. The three-step phase-shifting method and an optimal three-fringe selection method are applied to obtain the wrapped phase and unwrapped phase, respectively. A total of 9 color images are required to reconstruct the 3D shape of HDR objects. Compared to traditional methods, the proposed method has the advantages of reducing the number of projected images and improving measurement efficiency.
Research on pose calibration method for omnidirectional camera and rotation axis
Gao Yusen, Gao Nan, Ni Yubo, Meng Zhaozong, Shao Jinfeng, Zhang Zonghua
2023, 52(8): 20230425. doi: 10.3788/IRLA20230425
[Abstract](123) [FullText HTML] (14) [PDF 3231KB](39)
  Objective  Accurate determination of camera-to-reference frame parameters is crucial. Traditional systems are limited by camera field of view, constraining object size measurability. Omnidirectional cameras offer wide view and high imaging quality by using rotation systems or combining with LiDAR for scene modeling. This paper proposes a calibration method for omnidirectional camera and rotation axis. It uses omnidirectional cameras to capture rotation of QR code chessboards. A reliable mathematical model and nonlinear fitting optimize initial results for accurate parameter estimation. This method has low equipment requirements, considering board placement within the camera's view. The experimental results indicate that the average optimized reprojection error of this method can be controlled below 0.15 pixel, satisfying the requirements of experimental measurements and demonstrating promising application performance in various scenarios.   Methods  A reliable system is proposed to calibrate the extrinsic parameters between the camera and the rotation axis. A omnidirectional camera with a resolution of 4 000 pixel×3 000 pixel is utilized to capture the dual ChArUco calibration boards (Fig.2). For the extrinsic calibration, an algorithm is designed to fit the rotation plane and different methods for establishing the axis coordinate system are introduced (Fig.5). The accuracy of the system is evaluated using the distance from the optical center to the origin of the axis coordinate system (Fig.9) and the reprojection errors under different conditions (Fig.11).   Results and Discussions   In this method, the Perspective-n-Point algorithm is employed to determine the camera's optical center coordinates. Subsequently, a nonlinear least squares fitting technique is applied to fit the rotation plane and sphere of the optical center (Fig.8). The circularity fitting standard deviation for the intersection between the plane and the sphere is measured to be 0.021 8 mm, while the flatness fitting standard deviation is 0.030 1 mm. The range of distances from the camera's optical center to the axis is found to be 0.085 mm, with a standard deviation of 0.021 mm (Fig.9). Additionally, the maximum reprojection error between the experimental reference group and the other two control groups is 0.141 6 pixel (Fig.12), thereby validating the accuracy of the proposed method.   Conclusions  To address the issue of pose uncertainty between the camera and the rotation axis, this paper proposes a calibration method based on a omnidirectional camera and dual ChArUco calibration boards. The method captures multiple sets of images containing the dual targets to obtain the position information of the camera's optical center at each shooting position. By establishing a mathematical model for coordinate system transformation, the pose relationship between the camera and the rotation axis is computed and optimized, effectively suppressing the influence of random errors in the experiments. Experimental results demonstrate that the proposed method achieves sub-millimeter-level accuracy in the distance between the camera and the rotation axis, with an average optimized reprojection error controlled below 0.15 pixel. Compared to other methods, the method presented in this paper has lower system complexity, improved accuracy by use of two calibration boards, and effectively mitigates random errors caused by placement variations. The results indicate that this method exhibits good robustness and convenience, making it reliably applicable to shooting tasks in diverse scenarios.
Grapefruit-type three-core fiber sensor for trace water based on silver-silk protein-silver structure
Xie Yongpeng, Liu Shuo, Zhao Linwan, Zu Qun
2023, 52(8): 20230394. doi: 10.3788/IRLA20230394
[Abstract](89) [FullText HTML] (22) [PDF 2062KB](26)
  Objective   Water impurity in organic solvent strongly affects the process of chemical reaction. However, current methods for detecting trace water still suffer from disadvantages such as complex operation, high toxicity of experimental reagents, low detection sensitivity, and the inability to monitor in real-time. In order to overcome these challenges, a D-type fiber sensor which combines silver and silk protein in the visible wavelength range is designed. It is designed for high-sensitivity detection of trace water in organic compounds, and the performance of the sensor is optimized and analyzed.   Methods  First of all, the circular air hole in the middle of the fiber is replaced by the grapefruit type, which increases its size by about ten times (Fig.1), which can reduce the difficulty of the sensor in the actual production process with high efficiency. In addition, the lower metal silver film on the side profile of D-type fiber can form an energy channel with the nearby large air hole of grapefruit type, which promotes the energy leakage of the fiber core into the plasma and enhance the SPR effect. Secondly, a certain number of silver grating structures are added to the lower part of the top silver film, which can make the incident bright part confined to the slit cavity. Through reasonable structural design and size optimization, the local surface plasmon resonance (SPR) wave and surface plasmon wave can further resonate, thus realizing the enhancement of SPR phenomenon.   Results and Discussions   Theoretical analysis shows that compared to a single-layer silver structure, the double-layer silver structure on the surface of the D-type fiber sensor can induce strong localized light. In the subsequent size selection process, the initial thickness of the silk protein and the height of the air hole (Tab.1), the height of the silver gratings (Tab.2), the amount of the silver gratings (Tab.3), the spacing of the silver gratings (Tab.4) and the thicknesses of the underlayer silver film (Tab.5) are optimized to obtain the optimal sensing structure. Through the above optimization, the optimal structural parameters of the sensor are as follows: the height of the fiber air hole is 4.5 μm, the height of the silver grating is 10 nm, the number of the silver grating is 58, the spacing of the silver grating is 50 nm, and the thickness of the underlying silver film is 31 nm. Compared with other structures or methods of trace water detection equipment in sensitivity or detection limit performance, the results show that the detection performance of the grapefruit-type three-core fiber sensor for trace water based on silver-silk protein-silver structure is much better than the previous design.   Conclusions   Based on metal insulator metal structure and SPR principle, D-type fiber sensor is designed to detect trace water in organic matter. The sensing materials mainly use silver metal and silk protein, and the sandwich structure composed of silver metal can significantly enhance the local electric field, thus improving the sensitivity of the sensor. Silk protein has good optical properties and can quickly and accurately make reversible volume changes in response to external stimuli. Therefore, water absorption of silk protein can be calculated by its expanded volume combined with Darcy's law, and the water content in organic matter can be further obtained. The high sensitivity measurement of trace water in organic matter is realized, and the sensitivity can reach 1.39 nm/ppt (1 ppt=10−12). Moreover, the fitted line has an R2 value greater than 0.999, achieving the expected performance. And the sensor is designed to have a long service life, and is not affected by temperature.
Design of high-precision integrated temperature control system of spaceborne blackbody
Hei Huage, Li Xiaoyan, Li Lufang, Cai Ping, Chen Fansheng
2023, 52(7): 20220852. doi: 10.3788/IRLA20220852
[Abstract](113) [FullText HTML] (24) [PDF 2547KB](24)
  Objective   As space infrared technology advances towards high quantification, higher requirements are demanded for the precision of blackbody temperature control. Simultaneously, as spacecraft functionality becomes more complex, integrated design is necessary to reduce power consumption and weight. Traditional blackbody temperature control systems based on CPU or DSP are unable to meet the demands of high integration and high precision. To address this issue, this paper presents the design of a high-precision temperature control system for on-board blackbodies based on FPGA.  Methods   Temperature acquisition and control are performed using an FPGA as the core control unit, enabling multifunctional high-speed parallel processing. The blackbody temperature measurement module adopts a three-wire Wheatstone bridge to minimize the influence of wire resistance. In the signal conditioning section, a three-stage active filtering and amplification, composed of integrated operational amplifiers, is employed to achieve low-noise amplification of the electrical output. Compared to traditional instrumentation amplifiers combined with passive filtering, this method exhibits stronger interference suppression capabilities. Additionally, to address the non-linear error between platinum resistor resistance and temperature, as well as circuit errors in the temperature measurement system, a hierarchical fitting correction method based on polynomial models and least squares theory is proposed to further improve temperature measurement accuracy. The temperature control module incorporates a novel fuzzy control and incremental PID (FIPID) combination to reduce overshoot, accelerate convergence speed, and achieve high-precision temperature control.  Results and Discussions   Based on the measurement results using precision standard resistors, the temperature measurement accuracy of the system within the range of 247-375 K is 0.035 K, which is a 90.9% improvement compared to the uncorrected accuracy of 0.383 K (Tab.4). Temperature control simulation experiments demonstrate that compared to PID control, the FIPID algorithm achieves zero overshoot, while the PID algorithm has a 12.4% overshoot. Furthermore, the FIPID algorithm exhibits a 64% improvement in convergence speed (Fig.6). Ground thermal vacuum and on-orbit temperature control experiments indicate that the measured temperature control accuracy within the range of 256-367 K is 0.039 K, with a steady-state deviation not exceeding 0.018 K, and a temperature rise stabilization time of less than 10 minutes for a 10 K increase (Tab.5-6).  Conclusions   Traditional blackbody temperature control systems based on CPU or DSP cannot meet the requirements for high integration and high precision. To address this issue, this paper presents the design of a high-precision integrated temperature control system for on-board blackbodies based on FPGA. The approach uses a three-wire Wheatstone bridge to minimize the influence of wire resistance and introduces three-stage active filtering and amplification to improve the system's interference suppression capabilities. To mitigate temperature measurement errors, a hierarchical fitting correction method based on polynomial models and least squares theory is proposed. Additionally, a novel fuzzy control PID temperature control algorithm is introduced in the temperature control module to achieve high-precision temperature control. Experimental results demonstrate that the temperature measurement accuracy of the system is 0.035 K, which is a 90.9% improvement compared to the pre-optimized accuracy. Temperature control simulation experiments show that this method achieves a 64% improvement in convergence speed compared to traditional PID control, with zero overshoot, while the PID algorithm exhibits a 12.4% overshoot. Ground thermal vacuum and on-orbit temperature control experiments indicate that the measured temperature control accuracy within the range of 256-367 K is 0.039 K, meeting the requirements for on-orbit high-precision calibration and high integration. The system has been successfully applied to an on-orbit infrared camera of a specific model. The system possesses the advantages of high temperature measurement and control accuracy, wide dynamic range, and ease of integration, making it suitable for widespread application in other high-precision active temperature control systems in space.
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